A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z



The automatic adjustment of the eye lens curvature that results in a change of the focal length of the lens and brings images of objects from various distances into focus on the retina.

Allied Ophthalmic Personnel (AOP)

It is very difficult to provide a precise definition for the term AOP as these eye health personnel are characterised by different educational requirements, legislation and practice regulations, skills and scope of practice in different countries and even within a given country. Typically AOP include Opticians, Ophthalmic Nurses, Orthoptists, Vision Therapists, Ocularists, Ophthalmic Photographer/Imagers and Ophthalmic and Optometric Assistants, Technicians and Administrators.

Anterior Chamber

Fluid-filled space inside the eye between the iris and cornea.

Aqueous Humor

Clear, watery fluid that fills the space between the back of the cornea and the front surface of the vitreous, bathing the lens. Produced by the ciliary processes. Nourishes the cornea, iris, and lens and maintains intra-ocular pressure.


Binocular Vision

Blending of separate images seen by each eye into one composite image. This is performed in the visual cortexes in the brain.


A person is defined (ICD10) as blind if they have a visual acuity of less than 3/60 (20/400, 0.05) or have a visual field of the better eye no greater than 10° in radius around central fixation.

Blind Spot

Sightless area within normal visual field. Caused by absence of light sensitive photoreceptors where the optic nerve connects to the eye.


Catastrophic Spending

This occurs when out-of-pocket (OOP) payments for health services consume such a large portion of a household’s available income that the household may be pushed into poverty as a result.

Central Vision

The best part of a healthy eye’s vision; used for reading and discriminating colour and fine detail.


Vascular (major blood vessel) layer of the eye lying between the retina and the sclera. Provides nourishment to the retina’s outer layers.

Colour Blindness

Reduced ability to discriminate between colours, usually shades of green and red. It is quite often hereditary.


Light-sensitive receptor cell in the retina that provides sharp visual acuity and colour discrimination.


Transparent mucous membrane covering the outer surface of the eyeball.


The reduction of disease incidence, prevalence, morbidity or mortality to a locally acceptable level as a result of deliberate efforts; continued intervention measures are required to maintain the reduction.


Transparent front part of the eye that covers the iris, pupil, and anterior chamber and provides about 70% of an eye’s optical power.


The Cataract Surgical Coverage is the proportion of people with bilateral Cataract eligible for Cataract surgery who have received Cataract surgery in one or both eyes. (In the GAP, the CSC is measured at both the 3/60 and 6/18 level.)


The Cataract Surgical Rate is the number of Cataract operations performed per year per one million population.


DALY (Disability Adjusted Life Year)

One DALY can be thought of as one lost year of ‘healthy’ life. DALYs for a disease or health condition are calculated as the sum of the Years of Life Lost (YLL) due to premature mortality in the population and the Years Lost due to Disability (YLD) for people living with a health condition or its consequences.


Unit used to designate the refractive power of a lens.



Outward turning of the lower or upper eyelid such that it does not rest against the eyeball, but is pulled away. Creates corneal exposure with excessive irritation and drying.


Reduction to zero (or a very low defined target rate) of new cases of an infectious disease in a defined geographical area as a result of deliberate efforts. Elimination requires continued measures to prevent re-establishment of disease transmission.


Inward turning of the lower or upper eyelid such that the lid margin rests against and rubs the eyeball.


The study of the occurrence, distribution and patterns of disease in large populations, including factors that influence disease and the application of this knowledge to improve public health.


The complete and permanent world-wide reduction to zero new cases of an infectious disease through deliberate efforts; if a disease has been eradicated, no further control measures are required.

Extra-Ocular Muscles

The six muscles that move the eyeball (superior oblique, inferior oblique, superior rectus, inferior rectus, lateral rectus and medial rectus).


Fleshy structure covering the front of the eye, which protects it, limits the amount of light entering the pupil, and distributes tear film over the corneal surface.



A common term for Hyperopia.


The central pit in the macula that produces the sharpest vision. Contains a high concentration of cones but no retinal blood vessels.



The Global Action Plan was adopted at the 66th World Health Assembly as part of resolution 66.4. Its full title is Universal Eye Health: a Global Action Plan 2014 – 2019. The vision of the GAP is a world in which nobody is needlessly visually impaired; where those with unavoidable vision loss can achieve their full potential, and where there is universal access to comprehensive eye care services.

GBD Study

The Global Burden of Disease Study is the most comprehensive world-wide observational epidemiological study to date. It describes mortality and morbidity from major diseases, injuries and risk factors to health at global, national and regional levels.

GDP (Gross Domestic Product)

A measure of the size of a country’s economy. It is the sum of the products produced within a country’s borders, including products produced by foreign-owned enterprises.

GVP (Global Vision Database)

The overall goal of the Global Vision Database (GVD) is to develop and deploy new and improved evidence on the prevalence of blindness and vision impairment and its causes, on intervention coverage, to inform and influence global priorities and programs. Find out more about the Global Vision Database here.



A surgical mannequin used in Trichiasis surgery training.

High-Income Country

A country defined by the World Bank to have a gross national income per capita of $12,736 or more in 2014.


Health-Related Quality of Life is a multi-dimensional concept that includes domains related to physical, mental, emotional, and social functioning. It goes beyond direct measures of population health, life expectancy, and causes of death, and focuses on the impact health status has on quality of life.



The International Agency for the Prevention of Blindness is an alliance of civil society organisations, corporates and professional bodies promoting eye health through advocacy, knowledge and partnerships.


The International Centre for Eye Health is a research and education group based at the London School of Hygiene & Tropical Medicine, working to improve eye health and eliminate avoidable visual impairment and blindness, with a focus on low-income populations.


The International Council of Ophthalmology represents and serves professional associations of Ophthalmologists throughout the world. It works with ophthalmologic societies and others to enhance ophthalmic education and improve access to the highest-quality eye care in order to preserve and restore vision for the people of the world.

International Dollar (Int$)

A hypothetical unit of currency that has the same purchasing power in every country. Conversions from local currencies to International Dollars are calculated using tables of purchasing power parities, which are taken from studies of prices for the same basket of goods and services in different countries. International Dollars can be used to compare expenditures between different countries or regions.

Interquartile Range (IQR)

The Interquartile Range is a measure of variability, based on dividing a data set into quartiles. It is equal to Q3-Q1.

Intra-Ocular Lens (IOL)

A synthetic artificial lens surgically implanted to replace the eye’s natural lens as part of Cataract surgery.


The pigmented tissue lying behind the cornea that gives colour to the eye and controls the amount of light entering the eye by varying the size of the pupillary opening.


Lacrimal Gland

An almond-shaped structure that produces tears. Located at the upper outer region of the orbit, above the eyeball.

Lazy Eye

A common term for Amblyopia.


The eye’s natural lens. Transparent, biconvex intra-ocular tissue that brings rays of light to a focus on the retina, through flexing by ciliary muscles.

Low-Income Country

A country defined by the World Bank to have a gross national income per capita of $1,045 or less in 2014.



This is a small ,central area of the retina surrounding the fovea and is the area responsible for acute central vision.


The middle number of a data set. If a set is divided into quartiles it is the equivalent of Q2.

Middle-Income Country

A country defined by the World Bank to have a gross national income per capita of more than $1,045 and less than $12,736 in 2014.

Mild Visual Impairment

A person is defined (ICD10) as having mild visual impairment if they have a visual acuity of worse than 6/12 (20/40, 0.5) but equal to or better than 6/18 (20/70, 0.3).

Moderate Visual Impairment

A person is defined (ICD10) as having moderate visual impairment if they have a visual acuity of worse than 6/18 (20/70, 0.3) but equal to or better than 6/60 (20/200, 0.1).



A common term for Myopia.


An abnormal formation of blood vessels, usually in or under the retina or on the surface of the iris. This may develop in Diabetic Retinopathy or Macular Degeneration.

Normal Vision

A person is defined (ICD10) as having normal vision if they have a visual acuity of equal to or better than 6/6 (20/20, 1.0).


Neglected Tropical Diseases are a diverse group of communicable diseases that prevail in tropical and subtropical conditions in 149 countries and affect more than one billion people, costing developing economies billions of dollars every year.



An Ophthalmologist is a medical doctor who has been trained in ophthalmic medicine and/or surgery and who evaluates and treats diseases of the eye.

Optic Nerve

The largest sensory nerve in the eye which carries impulses for sight from the retina to the brain.


Optometrists are the primary healthcare practitioners of the eye and visual system who provide comprehensive eye and vision care, which includes refraction and dispensing, detection/diagnosis and management of disease in the eye, and the rehabilitation of conditions of the visual system. The WCO has adopted the broad competencies of dispensing, refracting, prescribing and the detection of disease/abnormality as being the minimum required for individuals to call themselves an Optometrist.


The diagnosis and treatment of defective eye co-ordination, binocular vision and functional Amblyopia by non-medical and non-surgical methods, eg glasses, prisms, exercises.


Peripheral Vision

The vision elicited from light rays falling on retinal areas away from the macula.


The utilisation of ultrasonic vibration to break up a Cataract, making it easier to remove.


The proportion or number of individuals in a population that has a disease or condition at a particular time (be it a point in time or time period). Prevalence is a proportion or number and not a rate.
In the Vision Atlas reference is made to two measures of prevalence:

The ‘Crude’ or ‘Raw’ prevalence is a simple measure that is expressed as a number or the % of a population that has a particular health condition; e.g. If 10 people in a population of 1000 were blind then the prevalence of blindness would be 1%. This type of prevalence is used to estimate the total number of people in the population who have a condition – the population may be for a local area, a country, region or global.

However the older an individual becomes the more likely they are to have many types of health condition, including most eye conditions. The crude prevalence measures the whole population regardless of the age structure of the population. This makes it difficult to compare the prevalence rates of different populations that have very different age structures; e.g. Higher-Income countries generally have a much higher proportion of more elderly citizens than the Lower-Income countries.
In order to compare prevalence rates between different populations, Epidemiologists use the ‘Age–Standardised’ or ‘Age-Adjusted Comparative’ prevalence. A world standard population has been developed by demographers which represents an average of all the different populations in different countries. The numbers of people found in a population survey with a particular health condition can be modelled as a % against the age structure of this world standard population. The age standardised prevalence should only be used for comparative purposes and not for estimating the number of people in a country or region with a particular health condition.


The variable-sized black circular opening in the middle of the iris which regulates the amount of light entering the eye.


QALY (Quality Adjusted Life Year)

One QALY equates to one year in perfect health. A QALY is calculated by considering both the quantity of life (how long you live for) and the quality of life (the quality of your remaining years of life).



Rapid Assessment of Avoidable Blindness is a rapid, population-based survey methodology on blindness, visual impairment and eye care services among people aged 50 years and over.


A test to establish an eye’s refractive error and the best corrective lenses to correct any vision impairment. A series of lenses in graded powers are trialled to determine which ones can provide the clearest, sharpest vision.


The light-sensitive nerve tissue in the eye that converts images from the eye’s optical system into electrical impulses which are then sent to the brain via the optic nerve.


The intensely light-sensitive, specialised receptor cells in the retina that work at low-light levels (helping night vision). A normal retina contains around 150 million rods.



The opaque, fibrous, protective outer layer of the eye (the white part) which is directly continuous with the cornea.


The Sustainable Development Goals were adopted by the UN General Assembly in September 2015. They include 17 Goals and 169 targets to bring about change in all countries and globally to end poverty, protect the planet and ensure prosperity. They integrate all three dimensions of sustainable development – the economic, social and environmental – and are built on a commitment to leave no-one behind.

Severe Visual Impairment

A person is defined (ICD10) as having severe visual impairment if they have a visual acuity of worse than 6/60 (20/200, 0.1) but equal to or better than 3/60 (20/400, 0.05).

Snellen Chart

The traditional test chart used for assessing visual acuity. Usually contains rows of letters in standardised, graded sizes, with a designated distance at which each row should be legible to a normal eye. Usually tested at 6m or 20ft.


Trabecular Meshwork

A mesh-like structure inside the eye at the sclera/iris junction of the anterior chamber. It filters aqueous fluid and controls its flow to the anterior chamber.


Visual Acuity

The ability to distinguish the detail and shape of objects.

Vitreous Humor

A transparent, colourless, gelatinous mass that fills the rear two-thirds of the eyeball, between the retina and the lens.

VLEG (Vision Loss Expert Group)

The Vision Loss Expert Group is an international group of mainly Ophthalmologists and Optometrists with experience in ophthalmic epidemiology. See here for more about VLEG.



The World Council of Optometry is an international membership organisation with a mission to facilitate the enhancement and development of eye and vision care world-wide. It does this through education, policy development and humanitarian outreach.


The World Health Assembly occurs annually, is attended by delegations from all WHO Member States and focuses on a specific health agenda prepared by the Executive Board.


Working through offices in more than 150 countries, World Health Organization staff work side by side with governments and other partners to ensure the highest attainable level of health for all people.

Our Supporters